Generally, a Bourdon tube is made of a flexible metallic plate by sealing a hollow flat tube in a circular shape. As one end of the tube is applied with a pressure, the other end, which is not fixed, moves in a direction that a curvature radius increases, and the movement degree increases or decreases depending on a size of pressure applied, a size, thickness, material and curvature radius of the tube. The apparatus for measuring pressure of fluid using the above Bourdon tube is called a Bourdon tube pressure gauge.
FIGS. 1 through 3 are cross sectional views illustrating a construction of a conventional Bourdon tube.
The Bourdon tube used for the pressure gauge is classified into a spiral type of FIG. 1, a C-type of FIG. 2, and a helical type of FIG. 3. According to the spiral type, a very thin and flat tube is curved in a spiral shape, so that a long and physically sensitive Bourdon tube is made for thereby increasing a movement distance of a free end. An indicator may be directly attached to a free end without using other application apparatuses. According to the C-type and helical type, as the movement distance of the free end is relatively short, it is needed to move the indicator based on amplification using a lever principle or a hand fan shaped pinion gear.
Here, the Bourdon tube pressure gauge comprises a Bourdon tube which is formed in a spiral shape and expands based on a pressure, and a needle shaft gear which rotates by means of a displacement gear and has a needle at its front end for indicating scales.
The Bourdon tube pressure gauge is designed to visually indicate the pressure based on the displacement of the Bourdon tube which expands and contracts in accordance with the pressure. Namely, as the needle shaft gear with a fixed needle via the displacement gear rotates based on the displacement of the Bourdon tube, the needle rotates for thereby visually indicating the pressure scales.
Since the Bourdon pressure gauge indicates a pressure scale using a needle based on an analog method, it is needed to make the outer diameter of the scale plate larger so that a small scale is visually seen. In particular, since it is impossible to obtain a digital value, an expensive digital pressure gauge is needed. A user, who uses a digital pressure gauge and is used to a conventional Bourdon tube pressure gauge, is impossible to well recognize a certain output value in a permitted pressure with respect to the output value. The above problem occurs since the conventional users are used to an analog Bourdon pressure gauge for long time periods.
So as to overcome the above problem, an electronic type pressure gauge using a conventional Bourdon gauge is disclosed in the US patent 2004/0093952 (electronic pressure converter, referred to conventional art 1), and there is a Korean patent application number 10-2004-0086196 (electronic type Bourdon tube pressure gauge, referred to conventional art 2).
According to the electronic pressure converter of the conventional art 1, a bar type magnet having N and S poles is prepared, and a sensor is disposed at an intermediate portion of the bar magnet. The GMR (Giant Magneto Resistance) sensor senses the rotation of the bar magnet. The sensor comprises two fixed resistors, and four variable resistors, so that two output values V1 and output values V2 (Sin, Cos) are outputted from one sensor.
FIG. 4 is a block diagram illustrating a construction of a GMR sensor in a conventional electronic type pressure gauge.
As shown in FIG. 4, the fixed resistor is adapted to compute an offset value. A pair of upper and lower resistors detect the sensing direction of the variable resistor in a horizontal direction, and a pair of the other upper and lower resistors are positioned at angles of 45° and 90° and sense the magnetic force of the magnet. A pair of the upper and lower resistors of the left side generate the sine value V1, and the upper and lower resistors of the right side generate the Cos value V2.
The fixed resistors provided at the center of FIG. 4 are arranged to set the offset values of the output value V1 and the output value V2. The output value V1 is obtained from the fixed resistor disposed at the center and the variable resistor disposed at the left side, and the output value is obtained from the fixed resistor disposed at the center and the variable resistor arranged at the right side. When there is not provided magnetic force, since the fixed resistor value and the variable resistor value are same values, the output value Vn is 0. The fixed resistor outputs values which does not change with respect to the direction of the magnetic force, and the variable resistor outputs variable values depending on the magnetic force. The above GMR sensor may have a plurality of sensors for detecting over 360°.
In addition, so as to obtain the offset value V1, it is needed to first compute the minimum value of V1 from the maximum value of V1 and is divided by 2.
(V1 offset=(Ve max−V1−min)/2) is performed.
So as to obtain the offset value V2, the min value V2 is obtained from the max value V2 and is divided by 2.
(V2 offset=(V2 max−V1 min)/2) is performed.
In addition, the gain value V2 is obtained by subtracting the min value V1 from the max value V1, and the obtained value is divided by the value obtained by subtracting the min value V2 from the max value V2.
V2 Gain=(V1 max−V1 min)/(V2 max−V1 min) is performed.
The actual rotation angle is obtained using the values V1 and V2.
Arctan((V1−V1 offset)/(V2−V2 offset)*V2 Gain) is performed.
The problems of the conventional art are as follows.
The GMR sensor is similar with the AMR sensor since it is used for a high resolution and reaction speed (Bandwidth), and the magnetic force is used in a range of mT (mill-tesla), but since the sensitivity and Hysteresis are so high. So, it is generally used for measuring the displacement,
1. Signal to Noise Ratio: SNR
As shown in FIG. 4, since the output values V1 and V2 are obtained based on the variable resistors, assuming that the SNR is 1, when one output value is intended to obtain from the Wheatstone bridge, the SNR is 2. According to the sensor structure of the conventional art 1, the SNR has only 50% performance as compared to the structure of the Wheatstone bridge.
2. Offset Value
The max and min values should be measured so as to obtain the output offset value. Since the GMR sensor does not recognize the poles N and S of the magnet, it is possible to obtain the max and min values based on the rotation of 180°+1°. Namely, it is needed to obtain the max and min values by rotating over 180° before every measurement, and the rotation degree of the Bourdon pressure gauge is converted into digital values for thereby obtaining the pressure values.
In this case, when it rotates over 180°, the errors occurring in the current magnetic force and the external temperature are corrected, and the pressure rotation value of the Bourdon tube pressure gauge is converted into digital signals for thereby decreasing the errors. According to another method, the product is assembled and rotated over 180°, and the max and min values are stored in an electronic unit and is used.
In the former case, works are complicated, and in the later case, the magnetic force of the permanent magnet is weakened as the time passes, so that it is impossible to correct changing output values and to correct when the output values change owing to the external temperature for each measurement Namely, the errors of the measurement values occur owing to the elapse of the time or the change of the external temperature, not by the number of uses.
The magnetization coil is attached to the sensor, and offset value is set by obtaining the max value from the coil “+” and the min value from the coil “−”. In this case, when the magnetic force weakens depending on the elapse of time, and in the case when the external temperature changes, it is possible to obtain corrected values. In case that there is provided a certain apparatus which is able to generate and maintain a constant temperature in the electronic unit, it is possible to more accurately correct the errors with respect to the changes of temperature. In this case, the manufacture cost and power consumption increase, but it is possible to obtain a desired performance in the course of development of high accuracy standard pressure gauges. However, the conventional arts do not disclose any technology for implementing the above magnetizing coil.
3. Output Value
FIG. 5 is a block diagram illustrating a sensing direction of the variable resistor of FIG. 4, and FIG. 6 is a graph of an output value based on the rotation angle of FIG. 5.
As shown in FIG. 5, when the resistance unit is formed using one sensor, sine and −sine values are outputted. In this case, unnecessary −sine value is outputted. As shown in FIG. 5, when the rotation angle is obtained using one sensor, the use angle is limited, and it is impossible to recognize 360°.
FIG. 7 is a block diagram of a sensing direction of the variable resistor when the resistance unit is formed using one sensor of FIG. 4, and FIG. 8 is a graph of an output value based on the rotation angle of FIG. 7.
When the resistance unit is formed using one sensor as shown in FIG. 7, it is possible to obtain sine and cosine output values. In this case, it is impossible to obtain an accurate position with only one sensor.
As shown in FIGS. 6 and 8, the rotation angles b and b′ are obtained. In more detail, since it is not sure to judge whether the output value a is an output value at the rotation angle b or not, or it is an output value at the rotation angle b′, it is impossible to compute the rotation degree with one sensor. In particular, it is impossible to recognize the rotation angle 360°.
One measurement range is about 120°, and the valid angle is about 90°. In more detail, when the distance of the variable resistor is distanced by a few of mm, and the sensor is manufactured, and one sensor is installed at the Bourdon tube pressure gauge, the measurement is performed in about 9 hour direction and 3 hour direction, so that the measuring angle is limited.
4. Rotation Degree Measurement
It is very limited to compute the rotation angle with arc tangent using the output value from the resistance unit provided in the GMR sensor. As shown in FIG. 7, it is possible to sine and cosine values using one GMR sensor, but when it is needed to recognize the rotation angle 360°, the arc tangent is useless. As shown in FIG. 7, since it is limited when obtaining the output value of FIG. 7, the arc tangent value obtained based on the same is limited as well.
5. Multiple Sensors
For easier description, it is assumed that three sensors are used. In case that the GMR sensor is used, at least three sensors are needed for measuring at 360°.
FIG. 9 is a block diagram illustrating an example that two output values from three sensors are amplified twice and used in a conventional art. FIG. 10 is a block diagram illustrating an example that two output values from three sensors are amplified using a 6-way multiplexor in a conventional art. FIG. 11 is a block diagram illustrating an example that one output value from three sensors is amplified using a multiplexer in a conventional art. FIG. 12 is a block diagram illustrating an example that one output value from three sensors is amplified using a multiplexer in a conventional art.
Namely, FIG. 9 shows a layout of the sensors, and FIG. 10 through 12 are block diagram of the sensors for measuring at 360°. The conventional art 1 does not disclose the detailed method like in FIGS. 10 through 12 except that a plurality of sensors can be adapted.
5-1) In the conventional art 1, since and cosine values can be obtained for measuring at 360°. One sensor is capable of sensing at 120° for thereby sensing at 360°. The conventional art 1 does not disclose a detailed method on the sensor arrangement and a method for computing the rotation angle.
5-2) The SNR of the conventional art 1 is lower 50% as compared to the Wheatstone bridge. So as to increase the SNR, it is needed to use multiple AMPs as shown in FIG. 10. so as to increase the SNR, since multiple amplifiers are needed, the cost is relatively increased. As shown in FIG. 12, even when the circuit (3-way multiplexer) is formed with lowest cost, the NSR is still lower 50% as compared to the Wheatstone bridge.
5-3) The conventional art 1 is sensitive to the offset value and the external error factor (temperature, magnetic force change). In case that three sensors are used, the conventional art 1 does not disclose a method for improving the error.
5-4) The construction cost of the conventional art 1 is relatively high. It may be changed depending on the purpose of the circuit and construction, six fixed resistors are totally needed in the interior of the sensor.
5-5) FIG. 9 shows a method for amplifying two output values from three sensors two times. In this case, the cost is relatively increased, and noises occur in the course of the amplification. The SNR becomes better two times as compared to when one sensor is used.
5-6) FIG. 10 shows a procedure that two output values from three sensors are selectively amplified using a 6-way multiplexer. In this case, the SNR has the same problems as when one sensor is adapted.
5-7) FIG. 11 shows a method for selectively amplifying via the multiplexor using one output value from three sensors. In this case, the SNR has the same problems as when one sensor is adapted.
The present invention, which will be described later, is basically designed to overcome the above problems of the conventional art 1 by dividing a circular magnet into NS poles and SN poles, and the sensor is disposed near the same, and sensor senses the rotation displacement of the circular magnet and outputs the same.
In conclusion, the conventional art 1 has a high GMR hysterisis, and the SNR is low owing to the structure of the sensor, and the measuring error owing to the external factors is high in the course of measurement. It is impossible to implement a 360° rotation recognition with only one sensor. When multiple sensors are used, the cost for increased parts becomes high, so that the total cost increases.
The conventional art 2 of the Korean patent application number 10-2004-0086196 (electronic type Bourdon tube pressure gauge) will be described.
The conventional part 2 is basically implemented by a sensing method using four poles of a magnet, not two poles. So, multiple sensors are disposed at an outer side of the magnet for detecting at 360° with a structure for recognizing the magnetic forces formed outside the magnet. The number of sensors is in proportion to the number of magnet poles.
The problems of the conventional art 2 will be described.
Since the magnetic force is recognized outside the magnet, it is too sensitive to external magnetic force for thereby increasing the errors in the course of measurement. Since multiple sensors are separated and arranged, when each sensor is manufactured, the accuracy decreases owing to an error occurring by the lot and an inherent error of each sensor. When the sensors are installed at the substrate, the accuracy decreases owing to the SMT error.
In particular, since multiple sensors are used, the unit cost increased. When the sensors are integrated into one sensor, the size of the magnet is limited, and the cost of the sensors is too high. When each sensor is integrated into one chip, an economical price is possible, and manufacturing error is less, but the error corrections with respect to the temperature correction and magnetic force changes can not be performed in the conventional art 2. So, a high performance product can not be manufactured.
The electronic pressure converter of the conventional art 1 and the electronic type Broudon tube pressure gauge of the conventional art 2 have the following problems since they use one sensor in addition to the above-described problems.
Namely, when a high performance sensing apparatus is implemented, the conventional art 1 uses a certain sensor SMR, and a high hysterisis is obtained based on the characteristic of sensor, and two output values are computed using the fixed resistors in the structure of the sensor, so that lower NRB is obtained, and 360° rotation angle is measured using sine and cosine values using one sensor. In addition, when setting an offset value, a method is not disclosed for correcting the errors with respect to the change of magnetic force and external temperature.
When multiple sensors are adapted, the detailed installation positions are not disclosed. When three sensors are used for 360° recognition using the above sensor, the parts except for the sensor increase, namely, the amplifiers or multiplexers increase. So, the unit cost becomes high.
In the case of the conventional art 2, it does not disclose methods for protecting an external magnetic force and overcoming the position error occurring when a multiple sensor are installed and the inherent errors (lot error, each product-based error) of each sensor and methods for offsetting the errors owing to the temperature difference of each sensor. When multiple sensors are integrated into one circuit, the size of the sensors may increase, so that the size of the magnet is limited. Since multiple sensors are installed, the number of parts increases. Since four poles are adapted, at least four sensors are used, so that the needed numbers of AMP and MUX is proportionally increased.